Remove forums from external.php RSS feed

[valdet]

Hi all,

I have external.php provider RSS feed enabled.

However, I need to disable threads from specific forums to show up on external.php.

I know that if you limit those specific forums from public viewing (disallow guests), the external RSS will not pull the feeds.
But I am interested to know how to remove forums from RSS, while still keeping them open to public viewing (guests allowed).

Any ideas/solutions are appreciated.

Thanks.
Val.